This invention relates to a process for the continuous preparation of isocyanates containing uretdione groups by the dimerization of polyisocyanates in the presence of compounds containing one or more hydroxyl groups with the addition of dimerization catalysts. The invention also relates to the use of the products of the process for the production of polyurethane plastics.
The preparation of polyfunctional uretdione isocyanates in the presence of dimerization catalysts is known (see Kunststoff-Handbuch, Volume VII, Polyurethane, published by Vieweg-Hochtlen, Carl-Hanser-Verlag, Munich, 1966, page 16).
In German Offenlegungsschrift No. 2,452,390 and U.S. Pat. No. 3,993,641, the dimerization reaction is carried out in a continuously operating reaction screw which can be cooled. In this method, the polyisocyanate which contains uretdione groups, e.g. dimerized 2,4-tolylene diisocyanate, is obtained in the form of a powder. For many types of processing, this powder is not sufficiently fine until it is ground down. This method of preparation also causes heavy wear in the screw due to abrasion, which can be recognized, for example, by loud squeaking noises in the operating part of the screw.
In the method described above, it is difficult to remove the considerable heat of reaction by means of the pulverulent product which has little thermal conductivity. If a pure dimer is to be obtained, certain, relatively low temperatures must not be exceeded. It will be obvious that in large machines this condition results in uneconomical throughputs. The addition of inert plasticizing agents proposed in U.S. Pat. No. 3,993,641 results in plastics which contain external plasticizers, with all the concomitant disadvantages such as exudation, a high content of extractable substances and end products which gradually become brittle.
A process has now surprisingly been found by which uretdione polyisocyanates whose end products do not have the disadvantages mentioned above can be obtained in a very finely divided form at high machine throughputs in a single operating step, using isocyanate reactive compounds as dispersing agents.